Sequestration, otherwise known as geo-sequestration or geological storage, involves injecting a material, such as carbon dioxide, directly into underground geological formations. Declining oil fields, saline aquifers, and un-minable coal seams may serve as potential storage sites. For example, CO2 may be injected into declining oil fields to increase oil recovery. The geological barrier that prevents upward migration of oil also may serve as a long-term barrier to contain the injected CO2. To inhibit leakage at the injection wells, or other wells where potential leakage can occur such as current or disused production wells and/or monitoring wells, isolating cement is provided in the annular region between the well casing and the subterranean formations.